Donald Trump won a string of Republican presidential contests on Saturday, edging closer to becoming the party's candidate in November's election as he attacked his likely opponent Joe Biden.

The former president won the Missouri, Michigan and Idaho caucuses comprehensively, continuing his clean sweep of states so far.

Mr Trump, 77, told supporters at a rally in Virginia that he was "on a rocket to the Republican nomination".

He is on track to secure it next week.

His last remaining rival in the race, former South Carolina Governor Nikki Haley, is still seeking her first victory and is without a clear path to the nomination given Mr Trump's commanding lead.

He is projected to win all of the delegates available in Missouri and all in Idaho, according to the BBC's US partner CBS news, as well as all of those remaining in Michigan. A third of delegates in that state were awarded earlier this week after a primary that Mr Trump won.

Delegates represent their state or district at the respective party's national convention, and decide who its presidential nominee will be. So far Mr Trump has secured 247 Republican delegates, according to CBS, far more than Ms Haley's 24.

In the Virginia capital of Richmond on Saturday, thousands queued for several hours to hear Mr Trump speak. He vowed to "win big" on Tuesday, when 15 states will choose their presidential candidate on a day that could put him within striking distance of the nomination.

"We got numbers today that were unbelievable," he told the crowd.

But his speech in Virginia - and at an earlier event in Greensboro, North Carolina - largely focused on migration at the US-Mexico border, a message which polls show resonates well with his base. In more than a dozen interviews with the BBC on Saturday, his supporters said the issue was among their primary concerns.

Sharon Roberts, whose son Sean died of a fentanyl overdose in 2018, said she feared an "out of control" border would lead to other families experiencing similar losses. "I'm 100% for Trump, because he'll get these borders closed," she said.

Mr Trump took direct aim at Mr Biden and blamed him once again for the border crisis, after the pair held duelling visits there last week. But he ramped up his attacks by airing a conspiracy that the current president was deliberately encouraging migrants to cross from Mexico.

"Biden and his accomplices want to collapse the American system, nullify the will of the actual American voters and establish a new base of power that gives them control for generations," Mr Trump said. Of the migrants and the Biden administration, he told supporters: "They're trying to sign them up to get them to vote in the next election."

His comments prompted a swift response from the Biden campaign. "Once again Trump is projecting in an attempt to distract the American people from the fact he killed the fairest and toughest border security bill in decades because he believed it would help his campaign," spokesman Ammar Moussa said, referring to a failed bipartisan immigration bill that Mr Trump vocally opposed.

A New York Times and Siena poll released on Saturday found 48% of American voters would support Mr Trump in a hypothetical match-up with Mr Biden, compared to 43% for the current president. It also found strong levels of dissatisfaction for Mr Biden in both parties.

At the Richmond rally, Mr Trump pointed to the poll and urged his supporters to "send a signal" on Super Tuesday.

The poll, however, showed 45% of voters would back Ms Haley in a contest with Mr Biden, with 35% supporting the current president. "I defeat Joe Biden by double digits in a general election match-up, while Trump is barely outside the margin of error," Ms Haley said shortly after it was published.

In his speech, Mr Trump criticised Ms Haley and called on her to step aside to allow Republicans to focus on the general election in November.

She has vowed to stay in the race until at least Super Tuesday. But several former Haley supporters at the Trump event said enthusiasm for her had waned in the face of repeated losses.

The former president, meanwhile, urged his supporters to deliver a decisive blow to Ms Haley on Tuesday. "We want you to get out there and vote in big margins," he said. "We want to send that little freight train going along, because the biggest day in the history of our country is November 5."